Fibre-Based Fabry-Pérot Resonators

In this project, we aim to realize Fabry-Perot resonators which are based on optical nanofibres. The main idea consists in integrating the entire resonant structure on the subwavelength-diameter waist of a tapered optical fibre, see Fig. 1. Such a monolithic Fabry-Perot resonator has several advantages. First of all, it is intrinsically mode-matched, meaning that light will simply have to be coupled into the unprocessed part the fibre in order to be perfectly coupled into the resonator. At the same time, the outcoupled light can conveniently be collected at the end of the other unprocessed part of the fibre. Therefore, in contrast to fibre-based Fabry-Perot resonators with an air gap, no alignment of the resonator itself will be required. Secondly, due to the extreme transverse confinement of the light field to a cross-section of the order of the optical wave-length squared, the mode volume of the resonator modes will be of the order ofl2L, where L is the length of the Fabry-Perot resonator, given by the distance between the two mirrors. Mode volumes of the order of 10–100 cubic wavelengths will thus be within the scope of our resonator concept. The access to the light field will be possible through the strong evanescent field protruding into free space around the nanofibre.

Figure 1: By integrating all the components of a Fabry-Perot resonator on the waist of a subwavelength-diameter tapered optical fibre, an intrinsically mode-matched resonator with a very small mode volume can be realized.

While our resonator is conceptually simple, its experimental realization requires implementing highly reflecting mirrors on a subwavelength-diameter nanofibre. This non-trivial task is best tackled with a high contrast photonic structure, or Bragg fibre grating (BFG), which relies on a periodic modulation of the nanofibre’s refractive index.